Lenses are used to focus light in a variety of applications. For example, micro-structures, such as cells, are observed with light focused in microscopes. Lenses focus light by bending it according to the law of refraction. The law of refraction states that the change in angle (refraction) of a light ray as it passes from one material into another material is related to a material property called the index of refraction and the angle the light makes with the interface between the two materials. The index of refraction is a measure of the speed of light in the material. Lenses that are used in microscopes are specifically designed to bend light which has been reflected by small specimens so that their images appear magnified.
Lenses that are used in microscopes are normally at least a few millimeters away from the specimen. Light from the lens travels through air before being focused on the specimen that is to be imaged. Because air has a relatively low index of refraction, the degree of features that can be seen on the specimen is limited. In view of this, oil, which has a medium index, is placed between the microscope lens and the specimen. This change in the index of refraction of the medium permits slightly smaller features to be seen on the specimen relative to those features on the specimen that are seen when air is the medium. This is called oil immersion microscopy.
In order to achieve even greater performance, light is focused through a high-index solid held in contact with the specimen. This is called solid immersion microscopy. In order to produce a suitable solid immersion lens, the lens must have a high index of refraction. One possible material for constructing this type of lens is silicon, whose index of refraction is approximately 1.41 as compared to 1.00 for air. In addition, the solid immersion lens must be held in contact with the specimen under examination without causing damage to specimens, such as cells.
Due to the limitations on resolutions obtainable with conventional optical lenses for applications such as microscopy, techniques have been developed to decrease the Rayleigh limit on transverse resolution d. The Rayleigh limit is generally understood as being the minimum distance that two particles may be separated and still be distinguished. The Rayleigh limit is given by δ=0.82λ/(NA) where λ is the wavelength and NA is the numerical aperture of the focusing objective (NA=n sin(θ), where n is the refractive index of the medium, and θ is the angle between the outermost rays focusing on the sample and the optical axis). Generally, the numerical aperture is a measure of the resolving power of the microscope objective. It is a measure of the optical performance of the system and concerns the specific ability to differentiate small features and the light gathering capability of the system. Whether the NA is considered to be low or high depends upon the type of optical system that is being used. For example, a high NA for a high powered microscope (e.g., 60× objective oil immersion lens) would be about 1.40.
Coherent light such as laser light can be used to precisely control the wavelength of illumination λ. One way to decrease the transverse resolution is to increase the index of refraction of the optical medium, such as by use of oil-immersion microscopy or use of a solid immersion lens.
If an SIL is placed in contact with the sample under examination, illumination can be more readily focused on it, and use of the high NA of the system allows efficient collection of the excitation light with high optical transmission efficiency and observation of the sample with a very high resolution. In most of the cases, the SIL is used primarily for near-field microscopy, where the air gap between the SIL and the sample oblige those who do not want to use evanescent waves to work with a NA smaller than one.
A problem with the SIL technology is the complexity of its manufacture. For example, a polished glass sphere provided with a sequence of progressively finer alumina powders, requires a polishing time typically of many hours. Furthermore, the result is not perfect, and the polished surface is slightly rounded. Moreover, known lens structures in SIL configurations involve objective lens sets that are self contained and thus are difficult to use in a manner that maintains the lens in immersion contact with the object under observation.
What is needed is a method for simple, inexpensive and rapid construction of microfabricated lenses, including solid immersion lenses, and a lens structure which is suited for low-cost, even disposable usage and for micro-size applications which conventional lenses have been unable to achieve.